1. Field of the Invention
The invention relates to a long-interval grating in an optical fiber. This type of grating is used especially to obtain a mode-coupling filter. Mode-coupling filters are commonly used to equalize the gains of optical power amplifiers.
2. Description of the Prior Art
The long-interval gratings that exist at present have periodic deformations of the core of the optical fiber. These deformations may consist for example of changes in the refractive index which take the form of lines in the core of the fiber. These lines cannot be seen from outside as the smooth surface of the sheath does not allow anything to be seen. However, these standard type gratings cannot be used to make high-performance mode-coupling filters. Indeed, the filters obtained are very imprecise and/or dependent on the polarization of the light.
These imperfections are related especially to the manufacturing methods used. The standard gratings are made by means of lengthy and painstaking photo-recording methods in which the optical fiber is irradiated with ultraviolet rays. For this purpose, the fiber is subjected to preliminary hydrogenation and then exposed to ultraviolet rays. A final step is then used to resorb the hydrogen. The resorption of hydrogen is very slow, so much so that the time taken to make a grating by this kind of method is very lengthy. The process generally takes a few hours. Furthermore, the resorption of hydrogen is a very difficult step which requires a great deal of attention. Indeed, the indices of the core of the optical fiber are very difficult to plan by computation as they depend essentially on the conditions of resorption of hydrogen. It therefore becomes very difficult to foresee the value of the filtering wavelength of the gratings manufactured according to this method since a shift is created, the value of which fluctuates according to the conditions of resorption of hydrogen between the computed values and the experimental values. Consequently, the filters obtained by this type of lengthy and painstaking method are not precise and reliable enough for the perfect equalization of the gains of an optical power amplifier.
Furthermore, it sometimes happens that the periodic defect created in the core of the fiber does not have a symmetry of revolution. In this case, the filtering function obtained becomes dependent upon the state of polarization of the light.